A dental composite material containing a polymerizable monomer, a filler, and a polymerization initiator is called a composite resin, and this dental material is most widely used today as a restorative material for repairing fractures of teeth and dental caries. Such a dental composite material is required to have the following properties. Specifically, as a cured product obtained after polymerization curing, the dental composite material is required to have sufficient mechanical strength and hardness to serve as a substitute for natural teeth, wear resistance against occlusion of teeth in an oral cavity, surface smoothness and gloss, color matching with natural teeth, transparency, etc. Furthermore, as a paste which has not yet been polymerized and cured, the dental composite material is desired to have ease of handling (high handling properties) for dental clinicians and technicians, for example, proper fluidity and forming property, no adhesion to dental instruments, no stickiness, etc.
These properties of the dental composite material are greatly influenced by the component materials, shape, particle size, and content of fillers used therein and by the combination of the fillers used together. For example, when an inorganic filler having an average particle size of more than 1 μm is used, the filling rate of the filler in the polymerizable monomer can be increased easily and therefore sufficient mechanical strength as a cured product and high handling properties as a paste can be obtained. The use of such an inorganic filler has, however, a drawback in that it is difficult to obtain satisfactory gloss even after final polishing, and even if satisfactory gloss is obtained, the gloss cannot be retained for a long time. On the other hand, when an inorganic ultrafine particle filler having an average particle size of 1 μm or less is used, the surface smoothness and gloss after polishing of the cured product and the gloss durability in the oral cavity are improved. The use of such an inorganic ultrafine particle filler has, however, a drawback in that when the inorganic filler is mixed and kneaded with the polymerizable monomer, the viscosity of the resulting paste increases significantly, which makes it difficult to increase the content of the filler. As a result, the mechanical strength of the cured product decreases, and the unpolymerized pasty composition becomes sticky, which reduces the handling properties. Furthermore, when an organic-inorganic composite filler obtained by mixing inorganic ultrafine particles having an average particle size of 100 nm or less with a polymerizable monomer, curing the mixture, and grinding the resulting cured product is used, the handling properties of the paste are improved, but the content of the inorganic filler in the cured product still is insufficient. Since the surface of the organic-inorganic composite filler forms a weak bond with the matrix, the mechanical strength of the cured product is not sufficient. Under these circumstances, it is difficult to increase the mechanical strength and the surface smoothness and gloss after polishing of the cured product and the handling properties of the paste in a balanced manner.
On the other hand, in recent years, dental composite materials have been required not only to have the above-mentioned properties but also to match natural teeth, that is, to have colors and optical properties such as transparency and a light diffusion property similar to natural teeth. Studies have been conducted to impart these properties to dental composite materials. JP 09(1997)-169613 A describes a dental composite restorative material containing a polymerizable monomer, a first filler, and a second filler. The first filler has a refractive index difference of 0.06 or less from the cured polymerizable monomer. The second filler has a refractive index difference of more than 0.06 from the cured polymerizable monomer, and has an average particle size of 1 μm or more. In this dental composite restorative material, the degree of diffusion D represented by the following formula (1) has a value of 0.002 to 0.3:D=(I20/cos 20°+I70/cos 70°/(2I0)  (1)where I denotes the intensity of transmitted light through a sample, and I0, I20 and I70 denote the intensities (the intensities of light) of the transmitted light measured at angles of 0, 20, and 70 degrees respectively with respect to the direction perpendicular to the sample plate (the incident direction of the light).
JP 09(1997)-255516 A describes a dental composite material containing a polymerizable monomer, one kind of filler, and another kind of filler. The former filler is obtained by aggregating inorganic filler particles having an average particle size of 0.01 to 1 μm and subjecting the resulting aggregate to heat treatment, and has a refractive index difference of 0.06 or less from the cured polymerizable monomer. The latter filler has a refractive index difference of more than 0.06 from the cured polymerizable monomer, and has an average particle size of 1 μm or more. The degree of diffusion D represented by the above formula (1) has a value of 0.002 to 0.3.
JP 2002-138008 A describes a dental curable composition containing a polymerizable monomer, and an organic-inorganic composite filler having a refractive index difference of 0.01 or more as an absolute value from the cured polymerizable monomer, and having an average particle size of 1 to 20 μm. In this dental curable composition, the degree of diffusion D represented by the above formula (1) has a value of 0.01 or more.
The dental composite restorative material described in JP 09(1997)-169613 A or JP 09(1997)-255516 A matches natural teeth very well because the combination of the polymerizable monomer and two kinds of fillers having different refractive indices imparts a light diffusion property to the cured product. In addition, the cured product of this dental composite material has sufficient polishability because the inorganic filler has a primary particle size of 1 μm or less. However, the use of two kinds of inorganic fillers having different refractive indices makes it difficult to obtain sufficient transparency as a dental material, and there is room for improvement.
The dental curable composition described in JP 2002-138008 A uses an organic-inorganic composite filler having a refractive index difference of 0.01 or more from the cured polymerizable monomer and having an average particle size of 1 to 20 μm, and the use of this filler imparts surface smoothness and gloss after polishing and a light diffusion property to the cured product. The use of the organic-inorganic composite filler, however, makes the bond with the polymerizable monomer as a matrix insufficient. As a result, sufficient mechanical strength cannot be obtained, and there is room for improvement.